1. Field of the Invention
The present invention relates generally to a disk medium, an information recording device and an information reproducing device, and more particularly to a disk medium, an information recording device and an information reproducing device which are capable of efficiently recording and reproducing security information, etc.
2. Description of the Related Art
There have hitherto been proposed a variety of methods of recording sub-information such as security information on a disk.
For example, according to the invention disclosed in JP 8-147767 A, a track for security data is separately provided in a track pitch region between spiral tracks for main information, and the security data are recorded on this security track.
According to this prior art, the security data are recorded in a region separate from that for the main data, and hence, even if the data are reproduced from the main data recording track and copied to other disks, the security data are safeguarded against simultaneous copying. The security performance can be enhanced as compared with a case where the security data are simply embedded in the track for the main data.
According to the prior art described above, however, there arises a problem in that a recording density of the main data on the disk decreases. Namely, when the track for the security data is provided between the tracks for the main data, it is required that a fixed gap be ensured between the tracks for the main data, and therefore the track pitch of the tracks for the main data can not be reduced. Hence, even when trying to increase the recording density of the main data, there is a limit.